In the field of video technology, the modulation transfer function, or MTF, has been used to describe the ability of a display system to pass spatial detail information (i.e., high frequencies). MTF is usually described as a percentage of the full output range of a display system, with full output being considered 100% modulation.
Traditionally, MTF has been used in cathode ray tube (CRT) performance specifications as a method to specify usable visual bandwidth. Since direct-view, color CRTs include a spaced phosphor structure and shadow mask of some type. MTF testing is used to characterize the highest frequency of modulation or alternating white and black transitions. Early MTF test systems have used sine wave modulation to determine the peak-to-peak contrast obtainable from the display device. As the modulation frequency increases and the spatial distance between peaks and valleys decreases, a point is reached where the average contrast of the display device eventually rolls off to middle gray. At that point, no discernable detail survives.
Many high resolution (small dot triad pitch) CRTs have carried resolution specifications based on the number of pixels discernable at a modulation percentage of only 10% or 20%. While MTF is a term mostly known by display engineers, it is useful for characterizing the resolution of an entire system including the display device.
In general, display devices reproduce high frequency details along the horizontal axis of the image, since television and graphics systems still use traditional raster scanning methods to create, transmit, and re-create visual images, regardless of whether information is conveyed in the analog or digital domain. Therefore, the highest frequency demonstrable by a display device is one half the system clock frequency for fixed-resolution devices and one half the highest frequency of modulation for a CRT device, which specifies a particular percentage of MTF result (e.g., 10%) for its resolution specification. At this frequency, the maximum number of black to white transitions along the horizontal scanning axis is discernable. This translates into a field of vertical alternating lines, or pixels.
Within the video processor, or drive system, illuminated horizontal lines require the processor to attain full output and hold that level for a long period of time. To alternate pixels along one horizontal line time, the processor must rapidly transition from black video level (0%) to full white (100%) in the shortest time possible. The speed with which these full pixel excursions are accomplished along the horizontal scanning axis characterizes the system's, or display's, video bandwidth.
In FIG. 1A, columns 100 represent pixels at the highest value that a given display system can provide (ideally, “white”) at the maximum displayable data rate, whereas columns 101 represent pixels at the lowest value (ideally, “black”). When viewed by row (e.g., rows 103), which corresponds to the linear data stream of the raster scanned video data, the pixels alternate between “white” and “black,” or the nearest values that the display system can achieve.
Conversely, a horizontal white line on a raster-scanned display, whether a CRT or fixed pixel array type display, is considered low frequency information. Since the white line lasts for the entire duration of one horizontal scan line, it represents non-alternating, continuously illuminated information having a comparatively low duty cycle when compared to alternating pixels over one horizontal scan line time frame. Such low frequency display information is illustrated in FIG. 1B, as rows of pixels with constant value.
With the aforementioned in mind, test patterns (e.g., fixed video frames) may be used during post-production testing to verify that display systems meet performance specifications. Those test patterns sometimes use patches (small areas or squares) of horizontal lines alternating white and black in the vertical scan direction next to patches of alternating vertical lines (alternating pixels) along the horizontal scan direction. Since each of these patterns are set to exhibit full white or full black level, they appear to be of equal brightness to the human eye when system bandwidth is flat (i.e., the gain of the display system is substantially uniform between the high frequencies and low frequencies represented).
As system performance in the high frequency domain begins to suffer, the high frequencies roll off and do not attain full output level in comparison to the adjacent low frequency alternating lines. The immediate indication to the observer of the test pattern is that high frequency performance of the system is lacking; and, as the problem persists, the high frequency performance degrades until the patch of alternating pixels appears to be a constant 50% gray level (e.g., a value halfway between black (0%) and white (100%)). Unfortunately, while the educated observer can tell that the display bandwidth is compromised, there is no metric for determining the level of compromise in system bandwidth or high frequency performance.